In-band control is a characteristic of network protocols with which data control is regulated. In-band control passes control data on the same connection as main data. Protocols that use in-band control include HTTP and SMTP. This is as opposed to Out-of-band control

Out-of-band control

Out-of-band control is a characteristic of network protocols with which data control is regulated. Out-of-band control passes control data on a separate connection from main data...

 used by protocols such as FTP.

Example

Here is an example of an SMTP client-server

Client-server

The client–server model of computing is a distributed application that partitions tasks or workloads between the providers of a resource or service, called servers, and service requesters, called clients. Often clients and servers communicate over a computer network on separate hardware, but both...

 interaction:

Server: 220 example.com

Client: HELO example.net

Server: 250  Hello example.net, pleased to meet you

Client: MAIL FROM: 

Server: 250 jane.doe@example.net... Sender ok

Client: RCPT TO: 

Server: 250 john.doe@example.com ... Recipient ok

Client: DATA

Server: 354 Enter mail, end with "." on a line by itself

Client: Do you like ketchup?

Client: How about pickles?

Client: .

Server: 250 Message accepted for delivery

Client: QUIT

Server: 221 example.com closing connection

SMTP is in-band because the control messages, such as "HELO" and "MAIL FROM", are sent in the same stream as the actual message content.